Mastering Multimeter: 33 Essential Tips

1. Correct Usage of Multimeter

To use a multimeter correctly, familiarize yourself with the dial. Adjust the two zero position knobs gently. Choose the correct terminals, and insert the red and black probes into the appropriate holes.

Rotate the switch to the correct gear, and ensure the range is selected properly for accurate readings. Look closely at the scale lines to read values accurately.

After measurement, switch the probes to the high voltage range. Regularly check the internal battery; deterioration can lead to electrolyte leakage. Store the instrument in a good environment, avoiding vibration, humidity, and weak magnetic fields.

2. Proper Use of the Ohm Range on the Multimeter

To use the ohm range correctly, you should know eight key points. Ensure the battery voltage is sufficient, and that there is no voltage in the circuit being tested. Select the appropriate range, with the needle pointing to the middle of the scale.

Every time you change the range, you must re-adjust the zero for resistance. Ensure good contact at the probe tips, and do not touch the probe ends with your hands. For measuring continuity in wires, use a range above one thousand ohms.

To test diodes, use different ranges for different resistances. When measuring transformer windings, avoid touching them to prevent electric shock.

3. Precautions When Measuring Voltage with a Multimeter

When measuring voltage with a multimeter, there are eight precautions to take. Be aware of the internal resistance of the multimeter, and ensure someone is monitoring the process. Connect the multimeter in parallel with the live circuit, and do not change the range while measuring.

For DC voltage measurements, clarify the positive and negative terminals of the circuit. When measuring AC voltage, ensure the power supply remains on during the measurement. For testing kilovolt high voltage, use specialized probes.

4. Method for Measuring DC Current with a Multimeter

To measure current with a multimeter, switch to the milliamp range, and confirm the positive and negative terminals of the circuit. The meter must be connected in series with the circuit. Select a higher range to minimize impact on the circuit.

5. Using DC Method to Identify the Start and End of Three-Phase Motor Stator Windings

For three-phase motor windings, use the DC method to determine the start and end. Set the multimeter to the milliamp range and use a DC power source. Connect one winding to the meter and touch the other winding to the battery.

Observe the needle movement; if it moves immediately, the positive terminal is the start. If it does not reverse, switch the connections and test the other winding in the same way.

6. Residual Magnetism Method to Identify the Start and End of Three-Phase Motor Stator Windings

For motors that have been operated, use the residual magnetism method to identify the start and end. After marking the output terminals of the three-phase windings, connect them in parallel. Set the multimeter to the milliamp range and connect to the common point.

Slowly rotate the motor shaft while observing the meter needle. If the needle does not move significantly, the three starts and ends are connected together. If the needle swings left and right, it indicates two starts and one end are connected.

Swap the connections of one winding and re-test until the needle stabilizes, indicating separate starts and ends.

7. Loop Method to Identify the Start and End of Three-Phase Motor Stator Windings

For motors that have been operated, use the loop method to determine the start and end. Connect the three-phase windings in a triangle configuration. Set the multimeter to the milliamp range and connect in series with the three-phase windings.

Rotate the motor shaft evenly while observing the meter needle. If the needle remains stable, the windings are connected at the start and end. If the needle swings widely, one winding is reversed.

Two connection points indicate the start and end of the windings.

8. Measuring the Speed of a Three-Phase Motor with a Multimeter

To measure the speed of a three-phase motor, use a multimeter. Open the motor junction box and disconnect the terminal connections. Set the multimeter to the milliamp range and connect to any one phase winding.

Rotate the rotor for one complete turn and observe how many times the needle moves. For a two-pole motor, the needle will move once, indicating a synchronous speed of three thousand RPM. For a four-pole motor, it will move twice, indicating a synchronous speed of one thousand five hundred RPM.

9. Testing the Ground Resistance of Household Grounding Protection Lines

To test the ground resistance of household grounding lines, set the multimeter to the voltage range and connect the electric stove to the phase and neutral. Measure the voltage at the stove terminal to calculate the operating current value.

Reconnect the stove to the ground line and measure the voltage again. The difference in voltage readings divided by the operating current gives the ground resistance value, with about five percent error.

10. Identifying Phase and Neutral Wires in Low Voltage AC Power Sources

In a low voltage three-phase four-wire system, identify the phase and neutral wires. Set the multimeter to the voltage range, with a scale of 250 volts AC. Connect one probe to the ground and touch the other probe to the power line.

If the needle deflects significantly, the probe is touching the phase line. If the needle barely moves, the probe is touching the neutral wire.

11. Testing the Polarity and Condition of a Diode

To test the polarity of a diode, set the multimeter to the kilo-ohm range. A low resistance reading indicates the diode is forward-biased. Connect the black probe to the anode and the red probe to the cathode.

A high resistance reading indicates the diode is reverse-biased. Connect the red probe to the anode and the black probe to the cathode.

To determine if the diode is functioning, check the forward and reverse resistances. A large difference indicates a good diode, while similar readings suggest failure.

Zero resistance indicates a short circuit, while infinite resistance indicates an open circuit.

12. Checking the Condition of High Voltage Silicon Rectifiers

To check the condition of silicon rectifiers, set the multimeter to the voltage range. Connect the rectifier in series with the multimeter and apply an AC voltage of 220 volts. Set the DC range to 250 volts and connect the rectifier in the forward direction.

A reading greater than thirty volts indicates a good unit; if the needle does not move, there is a fault. In AC mode, a reading of 220 volts indicates a short circuit, while zero indicates an open circuit.

13. Testing the Condition of Capacitors

To test the condition of microfarad capacitors, set the multimeter to the kilo-ohm range. Connect the probes to the capacitor terminals. The needle should swing left and right; the greater the swing, the better the capacitor.

If the needle does not move, the capacitor is open. If the needle drops to zero and does not return, the capacitor has failed.

14. Using a Digital Multimeter’s Buzzer Function to Test Electrolytic Capacitor Quality

To test the quality of electrolytic capacitors, use a digital multimeter. Set the switch to the buzzer function and connect the probes to the positive and negative terminals. A short beep indicates the capacitor is good, while a prolonged beep suggests a larger capacitance. If there is no sound, the capacitor is open.

15. Safety Regulations for Using Clamp Meters

When using clamp meters, remember the safety regulations. Testing high voltage circuits must be conducted by two people. Ensure the potential of the wire being tested does not exceed the clamp meter’s voltage rating.

Always wear gloves and stand on an insulated platform. Maintain a safe distance from any charged parts of the body. When measuring low voltage bus currents, ensure proper insulation.

Avoid using clamp meters on poorly insulated or bare wires.

16. Correct Use of Clamp Meters

When using clamp meters, select the appropriate model and specifications. Start with the maximum range for a rough measurement, then select the appropriate range. Ensure the wire is centered in the clamp jaws for accurate readings.

Once the clamp jaws are closed around the wire, do not change the range while measuring. Clamp meters for current and voltage should be used separately. Avoid inserting both wires into the clamp jaws simultaneously.

After each test, set the range to the maximum.

17. Techniques for Measuring Three-Phase Three-Wire Current with Clamp Meters

To measure three-phase three-wire current using clamp meters, apply Kirchhoff’s law. Insert one wire into the clamp to read the current of that phase.

Insert two wires to read the current of the third phase. Insert three wires, and if the load is balanced, the reading should be zero.

18. Techniques for Measuring Small AC Currents with Clamp Meters

To measure small AC currents using clamp meters, wrap the insulated wire around the clamp’s core. Divide the reading by the number of turns plus one to obtain the actual current value.

19. Detecting Phase Loss in Star-Connected Three-Phase Resistance Furnaces

To detect phase loss in three-phase resistance furnaces, use clamp meters. If the current values of two phase wires are below the rated current, and one phase wire shows zero, that phase’s resistance wire is burned out.

20. Locating Short Circuit Ground Fault Points in Low Voltage Distribution Lines

In lengthy low voltage distribution lines, identifying short circuit ground faults can be challenging. If a faulty phase wire is connected to an electric stove, use a clamp meter to measure the current along the line segment.

The point where current is absent indicates the location of the short circuit fault.

21. Testing Thyristor Rectifier Units

To test thyristor rectifier units, use clamp meters. Clamp the anode connection wire and observe the current reading on the meter. A zero reading indicates the component is not operational.

For three-phase components, balanced current values indicate normal operation. Severe current imbalance suggests inconsistent phase shifting. If AC components show faults, the rectifier transformer may be missing a phase.

22. Detecting User Phase Theft

If a user’s single-phase energy meter records low or no consumption, use clamp meters to check before or after the energy meter. Clamp the phase line and neutral line; if the reading is not zero, theft is indicated.

Measure the phase line and neutral line separately; significant differences in readings confirm phase theft.

23. Safety Regulations for Using Insulation Resistance Testers

When using insulation resistance testers, adhere to safety regulations. Testing high voltage equipment must be conducted by two people. Ensure the equipment is fully powered off and properly discharged.

When measuring line insulation, obtain permission from the other party. Ensure both circuits are powered off, and avoid testing during thunderstorms. When testing near live equipment, position yourself appropriately.

Maintain a safe distance and ensure monitoring to prevent electric shock.

24. Correct Use of Insulation Resistance Testers

When using insulation resistance testers, select the appropriate voltage level. Ensure the equipment is powered off and properly discharged before testing. Clean the surface of the equipment to remove any dirt.

Position the tester appropriately, away from electric and magnetic fields. Place it horizontally without tilting, and conduct both open and short circuit tests. Use two color-coded single-core soft leads that are well insulated and not tangled.

Ensure the terminal buttons are clearly identified and connected correctly. Rotate the crank clockwise to gradually reach a constant speed. The testing time is not fixed; record the reading when the needle stabilizes.

25. Precautions for Insulation Resistance Tester Testing

Remember eight key precautions when using insulation resistance testers. Do not touch the terminal buttons during the test. Avoid wiping the meter’s glass during the testing process.

When testing equipment’s insulation to ground, connect the ground terminal to the housing. For large capacitive devices, disconnect at rated speed. When testing electrolytic capacitors, connect the ground terminal to the positive terminal.

For repeated tests on the same equipment, it is best to use the same tester. Record the temperature during insulation testing. Avoid testing resistances above one hundred kilo-ohms, and do not use the tester as a common meter.

26. Connecting Diodes in Series to Prevent Equipment Discharge to Insulation Resistance Testers

Connect diodes in series with the insulation resistance tester. When testing large capacitive devices, this prevents discharge current from the equipment. This eliminates needle fluctuations, ensuring accurate readings.

After testing, stop rotating the crank to avoid damaging the instrument.

27. Methods to Increase the Terminal Voltage of Insulation Resistance Testers

For low voltage insulation resistance testers, connect them in series to measure insulation. The combined voltage level will increase, and the insulation resistance reading will sum accordingly.

28. Insulation Absorption Ratio of Power Transformers

To assess transformer insulation quality, use the insulation resistance tester. At around twenty degrees Celsius, start timing from the moment of measurement: check the reading at fifteen seconds, and the stable value after a few seconds.

The ratio of two insulation resistance values is termed the insulation absorption ratio. A value greater than 1.3 indicates good insulation, while below 1.3 suggests moisture.

29. Quickly Assessing the Condition of Low Voltage Motors

To assess low voltage motors, open the junction box for testing. Use the insulation resistance tester to check the minimum insulation value, with a baseline of eight mega-ohms at thirty-five degrees Celsius, decreasing by half for every ten-degree drop.

For every ten-degree increase, the minimum reading should exceed the value. Set the multimeter to the milliamp range and use the star connection method. Connect to any two phase heads and slowly rotate the shaft.

If the needle swings significantly, repeat the test three times. If consistent readings indicate the motor is good, otherwise it is not usable.

30. Testing the Quality of High Voltage Silicon Rectifiers with Insulation Resistance Testers

To test the quality of high voltage silicon rectifiers, connect the insulation resistance tester to the two leads grounded to the rectifier terminals. Measure the resistance in both forward and reverse directions; a significant difference indicates a good rectifier.

If the two readings are very close, the rectifier may have failed. If both readings are infinite, the rectifier is open. If both are near zero, the rectifier has short-circuited.

31. Testing the Quality of Self-Ballasted High Voltage Mercury Lamps

To test high voltage mercury lamps, use a kilovolt insulation resistance tester. Connect the two leads to the lamp terminals. Gradually increase the speed of the tester.

If the reading is less than half a mega-ohm, the lamp glows well. If it does not light up and the reading is zero, the lamp has a short circuit. If the needle indicates infinity, the lamp has an open circuit failure.

32. Testing the Quality of Fluorescent Tubes with Insulation Resistance Testers

To test the quality of fluorescent tubes, use a kilovolt insulation resistance tester. Set the multimeter to the voltage range with a DC of five hundred volts. Connect the leads in parallel, ensuring polarity is correct.

Connect the two leads to the ends of the tube. If the tube lights up at rated speed, below three hundred volts is normal. If it glows slightly, it indicates aging above three hundred volts.

If the tube does not light up at all, it indicates that it is damaged.

33. Testing the Condition of Fluorescent Lamp Starters with Insulation Resistance Testers

To test fluorescent lamp starters, connect the insulation resistance tester to the two leads. Gradually and gently turn the handle; the neon bulb should flash red.

If the starter is functioning well, otherwise, it is damaged.

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